Do not try to export after dismissing the dialog

I happened to notice the UI briefly locking up when testing my previous
changes on some of the Pandoc exporters. While the bigger mess I found
along the way is more than a little fix can handle, this stopgap measure
will at least stop us from running pandoc when it isn't needed.

manuskript/exporter/manuskript/plainText.py

@@ -84,6 +84,8 @@ class plainText(basicFormat):
         settings = settingsWidget.getSettings()
 
         filename = self.getExportFilename(settingsWidget)
+
+        if filename:
             settingsWidget.writeSettings()
             content = self.output(settingsWidget)
 
@@ -91,7 +93,6 @@ class plainText(basicFormat):
                 print("Error: No content. Nothing saved.")
                 return
 
-        if filename:
             with open(filename, "w", encoding='utf8') as f:
                 f.write(content)